This invention relates to rear wheel driven passenger automobiles having underhung carriages (meaning that the body is constructed with some portion of the body within the frame structure), an engine located at the front end, and a rear passenger compartment having a floor. In particular, this invention relates to such a passenger automobile in which a transmission tunnel runs centrally along the floor to accommodate a drive shaft.
In general, most modern front-engine rear-wheel drive automobiles, whether they are of unibody or frame construction, are designed to provide a maximum of internal space in the riding compartment. In addition, automobiles are also generally designed within certain height restrictions. To accommodate these competing objectives it is typical to have an underhung carriage which has the effect of maximizing the internal space in the automobile while maintaining an acceptable overall height. The underhung carriage results in the external hardware of the car, such as the drive shaft and exhaust pipes, being set into recesses in the body. An example of such a recess is the transmission tunnel, which is formed in the floor panels of the automobile. The recesses provide structural reinforcement, because they act as stiffening corrugations. This is especially true of the transmission tunnel.
In automobiles having underhung carriages, a typical transmission tunnel, for accommodating a conventional drive shaft, runs front to back centrally and results in a raised hump or bump which runs through the passenger space along the floor. The drive shaft connects the output shaft of the transmission at the rear of the engine with the pinion of the rear wheel differential. The rear wheel differential in turn transmits rotation and drive to the rear wheels. A splined slip yoke may be used to accommodate changes in the overall length of the drive shaft and is usually located on the output shaft of the transmission.
Typically, conventional drive shafts are one piece for automobiles having non-independent rear suspensions. In a one piece drive shaft universal joints are required at either end of the shaft, because as the rear suspension moves up and down to accommodate irregularities in the riding surface, so does the rear differential, and the shaft attached thereto. The transmission tunnel provided in the floor of the automobile must be large enough to accommodate this deflection of the drive shaft.
There are a few cars that have two piece drive shafts including extended wheel base limousines. Extended wheel base automobiles are made by taking a conventional full sized automobile, such as a Lincoln Town car, cutting the car in half and welding metal panels into the opening to provide the desired amount of increased length. In such applications, it is necessary to replace the shorter conventional drive shaft member with a longer two piece drive shaft assembly which still requires a transmission tunnel in the floor to accommodate it. At either end of the second segment of the drive shaft is located a universal joint.
In a two piece drive shaft, in automobiles having a non-independent rear suspension, the universal joints are necessary because the rear differential moves up and down with the rear suspension. Therefore, the rear end of the second segment of the drive shaft tends to ride up and down as the rear wheel suspension accommodates irregularities in the riding surface. This deflection of the drive shaft must be accommodated in the height of the transmission tunnel. To locate the two piece drive shaft in place a support is usually provided on the section of the shaft that does not move.
The transmission tunnel or hump that appears in the floor of the passenger compartment reduces the riding comfort of passengers, especially in the middle portion of the rear seat. Typically, such passengers are forced to rest their legs on the raised portion of the hump causing their knees to be uncomfortably high relative their seat. Alternatively, they can rest one leg on either side of the hump but this tends to impinge upon the foot space occupied by occupants on either side of the seat. In an extended wheel base car it is also typical to insert an auxiliary rear seat either facing forward or more commonly rearward. Typically full width bench seats, rather than bucket seats, are used. Two full width seats have seating capacity for six people, quite comfortably. However, the presence of the hump means that in practice, only four passengers can be accommodated comfortably.
The problem with the hump in the floor of the rear seat or the transmission tunnel is exacerbated in the case of such extended wheel base automobiles. With female passengers, in an extended wheel base car having opposed rear seats a modesty problem can be created by having the knees higher than the seat exposing undergarments.
Clearly it would be desirable, in conventional automobiles, and particularly in extended wheel base automobiles if the hump could be eliminated or substantially reduced in size or profile, without creating other problems, such as excessive vibration, an unacceptably low ground clearance or an unacceptable weakness in the body structure.